I am having a hard time installing my serpentine belt on my 3.8L. My problem is that the tensionner pulley is too small for the belt. **I am positive I have the right size belt**
Do you mean the belt is too large, and that the tensioner pulley doesn't tighten the belt?
What year/engine is the car, and what size belt are you using?
Are you using under drive pulleys, and if so, what combination (alternator only, or accessory as well)?